1. Field of the Invention
The present invention relates to helicopter landing gear, more particularly intended for light helicopters, comprising two skids each having a longitudinal support stretch for standing on the ground and which are connected to a front cross-piece and a rear cross-piece which are themselves attached to the structure of the aircraft by connecting devices, the rear cross-piece being fixed by the ends of its descending branches to the rear part of the said longitudinal support stretches.
2. Description of the Prior Art
Conventionally, landing gears with skids principally comprise four elements: two cross-pieces connected to the airframe at the top and two skids for contact with the ground connected to the bottom parts of the cross-pieces. Such conventional landing gears are described for example in the document U.S. Pat. No. 2,641,423 and in the document FR 1 578 594.
On landing, the impact energy is absorbed by elastic deformation and then by plastic deformation of the cross-pieces, essentially by working in a flexion mode. In general, the cross-pieces are made of steel tubes and the skids are made of aluminium tubes. The major disadvantage of this concept is the great rigidity of the system, which results in high acceleration factors during landings, difficult frequency adaptation, with respect to the phenomenon known as "ground resonance", and a rather high landing gear weight.
The purpose of the present invention is to overcome these disadvantages of the prior art and to obtain helicopter landing gear with a new design making it possible to reduce the said disadvantages significantly:
approximately 20% reduction in mass, PA1 simplification of the manufacturing and approximately 10% cost reduction, PA1 approximately 10% reduction in the load factor on landing, PA1 elimination of mechanical anti-ground resonance systems. PA1 absorption of energy corresponding to normal vertical impact speeds, PA1 critical landing speed resulting in residual deformation situated outside of the normal operating range, and PA1 sufficient flexibility to avoid the use of an additional anti-resonance system.
For this purpose, landing gear according to the invention, of the type defined at the beginning, is characterized in that each of the said skids has at the front an inclined transition zone with double curvature orienting itself transversely with respect to the said longitudinal support stretches which stand on the ground, above the plane of the latter, the two transition zones together constituting in this way an integrated front cross-piece offset either forwards or backwards with respect to the front delimitation of the plane of contact of the said longitudinal support stretches of the skids on the ground.
The rear part of this landing gear will have the actual function of skids: on landing, it will come into contact with the ground and principally will transmit the differential torsion between the front elements and the rear of the landing gear. The front cross-piece, which is thus integrated with the "skid" section, will contribute to the overall energy balance and will have, thanks to the flexion of the said transition zones, a predominant function of absorption of the forces generated during hard and sliding landings: it will prevent the complete collapse of the landing gear towards the rear when the sliding speeds are very high. It is understood that the general shape of this integrated front cross-piece, which is situated such that it protrudes in front of the skids or such that it is offset backwards, results principally from bendings of the tubes, in two different planes, on both sides of the landing gear (right and left).
The ground resonance behaviour is characterized in particular by the landing gear's roll stiffness. As the landing gear is fixed to the structure, for example by means of a single rear point and two front points, the torsional stiffness with respect to the pitch axis is principally obtained by the flexion of the front cross-piece. The variant according to which the points at which the front cross-piece stands on the ground are offset longitudinally towards the front of the aircraft with respect to the points of attachment to the structure of the latter has the advantage of allowing the roll functioning of the assembly to cause the front cross-piece to work both in torsion and in flexion instead of in pure flexion. A reduced roll stiffness results from this, which improves the ground resonance behaviour of the helicopter in the roll mode and this avoids any divergent phenomenon which would risk causing an accident.
According to another important characteristic of the invention, the assembly of skids and cross-pieces is constituted by aluminium tubes, this aluminium being characterized by an elastic limit of about 75% of the fracture strength and by a relative elongation at fracture at least equal to 12%.
Also, advantageously, the wall thickness of the tubes constituting the said front and rear cross-pieces is degressive between the central section of the cross-piece and its junction with the corresponding skid.
These various compromises make it possible to meet the size requirements of the landing gear and to satisfy the following three criteria: